Its kinda in the name, it supplies power
Generally its used to refer to things that take power from the wall and convert it into a low DC voltage that most of your electronics use like USB power supplies or the power bricks that feed laptops.
A Class 2 power supply feeds low voltage DC to some device downstream. The Class 2 refers to a National Electric Code(NEC) rating which limits the max power, voltage, and current that can be provided by the supply so that its outputs are relatively safe to touch which is a good feature for things like Laptop chargers where an average untrained person is going to have access to the live DC connector.
It could just be a transformer, which turns household AC into some (probably) lower voltage. Or it could be a transformer plus a rectifier and some smoothing circuitry, for turning AC into DC.
If you look on the box, it might have some text about its input and output. They often say “input 120VAC output 5VDC”, something to that effect.
The power supply provides usable electricity to the rest of the computer parts. The standard wall outlet has too high of a voltage for your more delicate components (like little capacitors, microprocessors, etc) so the power supply serves as a bit of a gatekeeper to make sure that the current from your mains electricity doesn’t fry your components. It’s a lot like the charging bricks that plug into the wall and use a USB cable to charge a cell phone.
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